1. Field of the Invention
This invention relates to a speaker to be mounted on a vehicle (hereinafter referred to “speaker for vehicle”), which is applicable to car-audio.
2. Description of the Related Art
As seen from FIG. 5, conventionally, a vehicle 1 such as a passenger car has various speakers for car audio. Generally, since a passenger in a room 2, such as a driver of the vehicle 1 basically listens to left-right two-channel stereophonic reproduction in a posture facing forward of the vehicle 1, speakers 3 and 4 are mounted in spaces in left and right front doors 5 and 6. However, since the door spaces are not so large, bass speakers are mounted in a space of a rear trunk room of the vehicle in a manner that they are opened in an upper rear tray 7. Since the directionality of an acoustic output is not so conspicuous at a low frequency, stereophonic feeling will not be deteriorated so much in monophonic reproduction based on left and right synthesis in a bass range. In case where front-rear and left-right four or more channel stereophonic reproduction is performed, left and right speakers are arranged in the rear tray 7 and rear doors 9 and 10 at the rear of the vehicle 1.
The speakers 3 and 4 mounted in the front doors 5 and 6 may cover the reproduction of a frequency range varying according to a system format of a car stereo, but may probably cover the frequency range of a midrange including the main part of a voice. Such a frequency range is the main part of the reproduced sound and is required to have high quality. In order to improve the sound quality of the speaker, it is necessary to suppress the vibration in the other portion than a diaphragm as well as to flatten the reproduction frequency characteristic and reduce the distortion.
FIG. 6 shows a structure corresponding to a speaker apparatus disclosed in FIG. 1 of JP-A-2002-152884. FIG. 6 shows a side-sectional view at the upper half and a side view at the lower half. A speaker unit 12 of the speaker apparatus 11 serves to suppress the vibration due to sound emission from a diaphragm 14 with the aid of a weight 13 mounted on the backside. A magnetic circuit 5 of an external magnetizing type generates a strong magnetic field in a magnetic gap through a ring-shaped main magnet 16 made of ferrite, a center pole 17 and a plate 18 so that the electromagnetic force corresponding to the current flowing through a voice coil 19 is generated in the direction of an axial line 19a, thereby driving a diaphragm 14 supported vibratably by a frame 20.
In order to suppress the outer leakage of magnetism, the magnetic circuit 15 also includes a cancel magnet 21, which is made of ferrite and magnetized in an opposite direction to the main magnet 16, and a shield cover 22. The center pole 17, plate 18 and shield cover 22 are made of a ferromagnetic material such as iron and also referred to as a yoke.
The outer periphery of the top end of the diaphragm 14 is supported by an edge 23 of the frame 20 whereas the base end of the center of the diaphragm 14 is supported by a damper 24 of the frame 20 that also has a function of damping vibration. The base end of the diaphragm 14 is coupled with the tip of a bobbin of the voice coil 19, its opening is closed by a dust cap 25. A gasket 26 is provided on the outer periphery so that the edge 23 is not crushed owing to pushing during attachment.
The weight 13 is made of e.g. iron, and has a mass e.g. 1.5 times as heavy as the mass of the entire speaker unit 12. The weight 13 has a schematically cannonball shape, and a flat end face on the front side and a streamline curve on the backside. The sectional shape of the weight 13, which is vertical to the axial line 19a, has a smaller diameter than the maximum diameter of the magnetic circuit 15. From the center of the end face on the front side of the weight 13, a boss 27 protrudes. The weight 13 is connected to the backside of a center pole 17 of the speaker unit 12 at only the tip of the boss 27. The weight 13 has a through-hole at the center which passes from the backside to the tip of the boss 27, into which a bolt 28 is inserted from the backside so as to be connected to the center pole 17. The bolt 28 is engaged with a screw hole formed in the center of the center pole 17. A flat washer 29 and a spring washer 30 provided at the head of the bolt 28 prevents the bolt 28 from loosening.
In the speaker unit 12, the magnetic circuit 15 and voice coil 19 constitute a converter for converting an electric signal into a mechanical vibration. The converter and diaphragm 14, which emits a sound wave on the front side thereof, are supported by the frame 20. The mechanical vibration converted from the electric signal by the converter is emitted as the sound wave into the ambient air from the diaphragm 14. The counterforce applied to the diaphragm 14 from the ambient air returns to the converter. However, since the weight 13 having a larger mass than that of the speaker unit 12 is secured to the converter, the weight 13 serves as a virtual ground and hence the vibration is suppressed.
The speaker 3 and 4 as shown in FIG. 5 usually has a structure in which the frame of the speaker is screw-secured to a wall of a vehicle body, the wall defining a passenger room 2. In this structure, the magnetic circuit which generates counter force against the motion of the diaphragm for generating sound is likely to vibrate. This attenuates the efficiency of transmitting energy from the diaphragm to the ambient air, thereby deteriorating the sense of speed which is a transient characteristic of the reproduced sound.
If the speaker apparatus as shown in FIG. 6 is employed as the speaker for vehicle 3, 4 of FIG. 5, it is expected that the sense of speed is improved and hence the sound quality is improved. However, there is possibility that the front doors 5 and 6 and rear doors 9 and 10 do not necessarily have their sufficient depth enough to accommodate the weight 13 as shown in FIG. 6.